1. Field of the Invention
The present invention relates to hand-operated tools and, in particular, to pliers that may be self-adjusted for size and locked on a workpiece, and have the gripping force subsequently increased.
2. Description of Related Art
It is desirable to be able to lock pliers so that the user may release his hands from the pliers while the tool remains firmly gripped about a workpiece held in the jaws. Vice grip-type pliers, as shown in U.S. Pat. Nos. 2,514,130 and 3,600,986, may be locked about a workpiece. Self-adjusting pliers have been known in the prior art, as exemplified by U.S. Pat. Nos. 4,662,252; 4,651,598; 4,802,390; 4,893,530; 5,060,543; 5,351,584 and 5,408,904. While some self-locking features have been disclosed, the design of such features has provided potential problems in manufacture and use. Moreover, whether with vice grip or self-adjusting pliers, after the jaws have been locked in place it is often desirable to be able to increase the gripping pressure of the jaws on the workpiece to a degree beyond what the user may achieve by squeezing the handles.